In various drilling situations it is desirable to drill at an angle from the vertical. Such situations include drilling inaccessible locations (such as under rivers or other bodies of water when the drilling begins on land), drilling around obstructions which prevent a substantially vertical well direction and, the use of drain holes in which the bore hole penetrates substantially horizontally through a relatively thin subterranean stratum to drain the fluids therefrom more effectively than would be possible with a vertical bore hole.
In drilling, a "sub" is a short threaded piece of drill pipe generally used to connect parts of the drill string which cannot otherwise be screwed together because of difference in thread size or design. One manner of achieving directional drilling is to insert, at the downhole end of a drill string, a sub which has been "bent" such that the longitudinal axis at one of its ends is at a slight angle, referred to herein as "offset angle", to the longitudinal axis at the other of its ends. Such a tool is called a "bent sub". To vary a bore hole away from the vertical axis, the drill string is withdrawn and a bent sub having the desired offset angle is inserted between the end of the drill string and the downhole motor. The drill string is then reinserted into the hole and, as the longitudinal axis of the drill bit will now be at an angle to the original bore hole, the direction of the bore hole will be altered. The bent sub may be replaced any number of times in order to provide a bore hole of the desired shape.
It will be appreciated that the more remote the bent sub is from the drill bit, the lesser will be the maximum offset angle before the drill string will scrape against a bore hole. One method of moving the bent sub as close as possible to the drill bit, and hence maximizing the allowable offset angle, is to position the bent sub between the power unit and bearing components of the downhole motor.
A disadvantage with using a bent sub as described above is that to make any angle corrections while drilling is in process, it is necessary to raise the complete drill column out of the drill hole, disassemble the drill column, remove the sub, replace the sub with one having a different offset angle, and reinsert the drill column into the drill hole. To reduce the down time in such a process, various adjustable bent subs such as that described in U.S. Pat. No. 4,745,982 issued to Wenzel have been developed. While these subs eliminate the down time attributable to disassembling the drill column, they still incur the down time associated with removal and installation of the drill column from the drill hole.
To reduce this latter down time, various adjustable bent subs have been developed which are "downhole adjustable" in that they can be adjusted without removal of the drill column from the drill hole. Examples of such downhole adjustable bent subs include U.S. Pat. Nos. 4,286,676, 4,596,294 and 3,811,519 which issued to Nguyen et al., Russell, and Driver respectively. Nguyen and Russell teach the use of subs having at least one swivelable angled joint. Relative rotation of the parts of the sub on either side of the angled joint used in Nguyen and Russell causes the offset angle to vary. Both Nguyen and Russell have rotating mechanisms inside of the sub which react to drilling mud pressure to effect the rotation. Driver teaches the use of a plurality of radially disposed hydraulic cylinders to bend a flexible section of the drive shaft connecting the downhole motor and the drill bit.
A disadvantage with Nguyen is that to effect rotation, either electrical connection must be made down the drill string to the adjustable sub, or spheres must be pumped down to the sub through the drill string. Both of these variations of Nguyen interfere with the flow of drilling mud through the drill column.
The device in Russell is actuated by successive increases and decreases of internal drill string pressure. This device relies on a combination of gas and spring pressure to control a rather complicated spherical valve which in turn activates the rotating mechanism. The complexity of this device gives rise to many possible sources of failure.
Driver requires that hydraulic lines be extended to the hydraulic cylinders. This would interfere with the flow of drilling mud, make the connection of adjacent sections of drill string more time consuming and give rise to sources of possible failure.